Nothing, as long as it is kept personal.
I refer to many different resources in my own personal study. However if I came on the board and every post said: But Spurgeon says:" and again, But Spurgeon says: " But Spurgeon says, " etc., Then who is my authority? Is it Spurgeon or the Bible? Spurgeon believes the Bible. But if I constantly quote him as my authority I have allowed him to become my authority rather than my Bible. Some people have done that with Calvin, with the ECF, and some with Confessions. Let us make sure that our authority is the Bible, all the time!
Again, it is not wrong to refer to other resource material, but let us not make it our authority by continually doing so, or habitually doing so. The Bible must be supreme. "But sanctify the Lord Jesus Christ in your heart and be ready always to give an answer to every man (from the Bible) that asks of the hope that is in you with meekness and fear."
Yes, I added "from the Bible" because that is what Peter had in mind. He wasn't speaking of Augustine, Calvin, Confessions, etc. He was referring to the Word of God.
Understand your concern, as I at times have wondered why so much quoting from Confessions/creeds/authors instead of the bible...
other concern is that at times its almost like if the doctrine is not found in those items, its not biblical!